Like Soberleigh said, being nervous and scared sounds like a good thing to me. It means that deep down you're serious about not drinking at all on this trip. Which is fantastic to hear.
The advice you've had so far has all been good. It does sound like your BIL is the big risk. But remember, there is no reason whatsoever why you have to drink with him, no matter how much he wants that. Don't head off to a bar with him planning to stick to sodas. Don't let yourself end up alone with him late at night with a bottle of whiskey (or whatever) on the table and hope your willpower holds out. His alcohol problem is his alcohol problem. The best way you can help him right now is to show him that you're strong, you're not drinking, you've changed how you live, and hopefully you can be an inspiration that maybe he could do it too. He can't make you drink. That's your choice. And you've made the choice to be sober.